Hi. I use Morrisons Self Raising Flour is. On the package, it says it is Produced in the UK. It doesn’t say if the ingredients are British though.

I also use Allison, a British brand/company, founded by Thomas Alison in 1892. Currently I have several packages of Bread Flour, but can’t figure out if it is made out of British ingredients. Their website states “Most of the grain is grown in the UK. However, some comes from North America and continental Europe.” Being such an old brand, it might be worthwhile to push them to go a bit further in their packaging info and request for the ingredients origin to be disclosed in each speciality flour.
Website: http://www.allinsonflour.co.uk/products/how-we-make-our-flour.html

I also use some flour from Doves Farm, which is a British brand/company. Unfortunately, I can’t find info on the origin of the ingredients. I have the feeling most of it might be British, but it would be another case worthwhile investigating.
Website: http://www.dovesfarm.co.uk/about/about-us/